1. Field of the Invention
The present invention relates generally to the manufacture of nuclear fuel rods to be incorporated into fuel assemblies for nuclear reactors and, more particularly, is concerned with an improved apparatus for applying an end plug to a fuel rod tube end such that the end plug is guided in a secured manner into the tube end regardless of where the end plug falls in its diametral tolerance range.
2. Description of the Prior Art
Fuel elements or rods for nuclear reactors commonly encase the fissible material in thin walled cladding or tubes which serve to support the nuclear fuel during the operation of the reactor. The nuclear fuel, which is usually in the form of cylindrical pellets of enriched uranium dioxide, must be isolated from the environment surrounding the tubes to prevent contact and chemical reactions between the fuel and other materials such as water in a pressurized water reactor. Thus, the nuclear fuel is ordinarily hermetically sealed in each thin walled tube by the use of a pair of opposite end closures or plugs.
It is critical that the end plugs themselves be impermeable and mechanically strong to contain the fissible products. It is equally critical that the mechanical connection of each end plug with an end of each tube be free of defects such as discontinuities, cracks and tube distortions which could eventually produce leaks.
Present methods of applying the end plugs use close fitting guides to align the plugs with the tube, such as illustrated in FIG. 2. Due to manufacturing tolerances, the necessary clearance between the plug and guide sometimes permits the plug to cock, or tip, as shown in solid line form in FIG. 2, which causes a shaving action on the plug exterior as the plug is rammed home. Thereafter, when the connection is completed such as by welding the plug to the tube, faulty welds commonly result which produce leaking because of discontinuities in the weld joint or connection caused by this shaving action. Also, frequently the plug does not seat properly which increases the likelihood of leakage. Additionally, pieces of the plug are broken off occasionally as it is forced into the tube. The broken pieces not only damage the end plug but also cause foreign objects to be deposited in the fuel rod.
Consequently, a need exists for an improved technique for applying an end plug to the tubular end of a fuel rod which will accommodate slight variations in the dimensions of interfitting parts due to unavoidable manufacturing tolerances and thereby increase the resistance of fuel rods to leakage and failure. SUMMARY OF THE INVENTION
The present invention provides several embodiments of an improved apparatus for applying an end plug to the end of a fuel rod tube in a manner designed to satisfy the aforementioned needs. In each embodiment, intimate contact is maintained with the end plug by a conformable plug guide means which results in the end plug being securely guided until it enters the hollow tube end. The maintenance of continuous contact between the guide means and the end plug prevents the end plug from being propelled forward out of contact with a ram member being used to move the end plug through the guide means and into the tube end. As a result, the end plug is prevented from cocking, tipping or tumbling within the guide means so that it would not meet the tube end squarely.
Accordingly, the present invention sets forth an improved apparatus for applying an end plug to a hollow tube end of a nuclear fuel rod such that a close frictional interfitting engagement will be established between the end plug and the tube end facilitating the formation of a connection therebetween which hermetically seals the fuel rod, the improved apparatus comprising: (a) a housing having spaced inlet and outlet ends, the inlet end adapted to receive the end plug and the outlet end adapted to receive the tube end to which the plug is to be applied; (b) guide means disposed in the housing and having internal surface portions which define an internal guide channel aligned in tandem with the inlet and outlet ends of the housing along a common axis and being of a cross-sectional size smaller than that of an outer external surface of the end plug, the guide means being yieldably expandable radially with respect to the common axis; and (c) means disposed adjacent the inlet end of the housing and movable along the common axis for engaging and moving the end plug from the inlet end through the guide channel to the outlet end, the movement of the plug through the guide channel causing yieldable expansion of the cross-sectional size of the guide means such that the surface portions of the guide means conform to the external surface of the end plug and maintain guiding contact therewith as the end plug is moved through the guide channel.
In a first alternative embodiment of the apparatus, the guide means is a bushing disposed in the housing which is composed of resiliently deformable material adapted to expand and conform to the outer external surface of the end plug upon contact therewith and thereby establish and maintain guiding contact with the end plug as it is moved through a central bore of the bushing which defines the guide channel.
In a second alternative embodiment of the apparatus, a plurality of elongated runners are mounted for radial movement along a series of passageways which are defined in the housing in spaced relationship to one another and extending generally parallel to and radially from the common axis. A plurality of resiliently expandable members circumscribing the housing and the runners bias them inwardly toward one another. However, the members are yieldable to allow movement of the runners away from one another upon contact with the outer external surface of the end plug for establishing and maintaining guiding contact with the end plug as it is moved through the guide channel defined by inner end surfaces of the runners.
In a third alternative embodiment of the apparatus, a plurality of elongated generally cylindrical rolls are mounted for movement radially in a series of recesses which are defined in the housing in spaced relationship to one another about the common axis and aligned generally parallel to one another and with the common axis. A plurality of resiliently expandable elements encircling the housing within respective circumferential slots therein which intersect with the elongated recesses and stretched about the rolls bias them toward inner ends of the recesses. However, the elements are yieldable to allow the rolls to move away from the inner ends of the recesses upon contact with the outer external surface of the end plug for establishing and maintaining guiding contact with the end plug as it is moved through the guide channel defined by inner facing surface portions of the rolls.
These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.